lin-13 has been identified in screens for mutants defective in C. elegans vulval lineages. It functions to restrict three of the six equipotential vulval precursor cells from undergoing vulval cell fates. Genetic analysis suggests that existing alleles only partially reduce gene function, and that the complete loss of function (null) phenotype may be lethality. So, lin-13 may also have an essential role in development. I propose to study the roles of lin-13 in development using a combination of genetic and molecular analysis. To identify the role lin-13 plays early in development, null alleles will be isolated and characterized. The terminal phenotype of these alleles will be examined, and any cell lineage transformations will be described. Genetic mosaic analysis will be done to determine where in the cell lineage lin-13 activity is needed for viability and for vulval development. Genes that interact with lin-13 will be characterized in two ways: the construction of double mutants with lin-13 and other genes known to function in the vulval pathways; and the isolation and characterization of suppressor mutations. Finally, to begin investigating the molecular mechanisms of lin-13 action, the gene will be cloned and sequenced.